1. Field
This invention pertains to collision threat assessment for vehicles, such as aircraft, to enable timely action, when necessary, for avoidance of other aircraft or suitably designated obstacles.
2. Prior Art
Many collision avoidance systems (CAS) have been devised or proposed for providing an estimate of the time to a collision, usually designated as TAU. Such systems generally are intended to operate by measuring the range between the Own aircraft and the Other aircraft, and measuring or computing the range-rate, or radial closing velocity. TAU is simply the range divided by the range-rate, and when it reaches some predetermined value, say 20 seconds, it may be assumed that some avoidance maneuver should be undertaken.
Airborne radio measurements of range and range-rate usually require two-way transmissions between the Own aircraft and each Other aircraft in the general area, resulting in the so-called N.sup.2 problem, with excessive jamming or "fruit." Ground-based measurements require data uplink transmissions to at least any aircraft approaching a collision situation, and can be subject to intolerably large errors under certain conditions.
Proximity indicators or warning systems (PWI) that use the existing standard Air Traffic Control Radar Beacon System (ATCRBS) signals to minimize or avoid two-way air-to-air transmissions are described in the following U.S. Patents:
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These systems take advantage of the ATCRBS signal format and the azimuthal scanning characteristics of the Secondary Surveillance Radar (SSR), which are standardized throughout the world, without interference with their present functions and without requiring additional frequency assignments.
A PWI by definition responds to an existing proximity situation rather than a potential one. Accordingly, the protected airspace, i.e., the volume surrounding the Own aircraft within which another aircraft is declared to be in proximity, must be large enough to allow time for appropriate action before an actually unsafe proximity can occur. PWIs do not provide a threat assessment, leaving the pilot to do so. The required volume must be large enough so that at least occasionally it encloses other aircraft on such courses and/or moving at such relative velocities that they offer no possible threat of collision with Own aircraft. Such aircraft will produce unneeded proximity warnings, frequently when they are most troublesome.